As is well known in the art, there are two basic types of semiconductor wafer stepper systems: an on axis stepper system wherein alignment is accomplished directly through the lens of the stepper system and an off axis stepper wherein alignment is performed through a separate microscope alignment apparatus spaced from the lens column of the stepper system. On all reticles, whether utilized in an off axis or on axis stepper system, there is an alignment inspection aid that is designated as the aid to be used to gauge total overlay of the pattern to be aligned with a previously processed layer on a wafer. The alignment at the lower right hand section of the wafer (with the wafer flat upward) at the die cluster that is directly below the right hand microscope objective (the baseline objective) while the wafer is at the align position, is commonly referred to as the baseline (or X, Y) alignment. The second microscope objective views the die cluster used for angular, or theta, alignment.
The baseline error is the distance from the actual position of the right side microscope objective to the nominal location of the right side objective. Corrections made in software allow the stepper to know the baseline distance a wafer must travel after accurate alignment to reach the optical axis of the reduction lens (i.e. the center of the reduction lens) of the off axis stepper system.
Placement of alignment marks and alignment inspection aids are used universally on reticles of stepper aligners. Generally, they are placed in the scribe grids in a manner that does not attempt to minimize the impact of field registration errors on their subsequent image transfer. The contribution of baseline errors due to field registration errors is impacted by both the placement of the alignment mark and the alignment inspection aid.
It is an object of the present invention to provide new and improved reticles for semiconductor wafer stepper systems.
It is a further object of the present invention to provide improved reticles for semiconductor wafer stepper systems wherein alignment marks and alignment inspection aids are positioned to greatly reduce alignment errors.
It is a further object of the present invention to provide a method of determining software corrections to provide correct baseline distances for off axis alignment stepper systems
It is a further object of the present invention to provide a method and apparatus for improving registration accuracy in semiconductor wafer processing and to reduce reprocessing due to misalignment.
These and other objects of this invention will become apparent to those skilled in the art upon consideration of the accompanying specification, claims and drawings.